From 3fa7d9d1fad9cfc100aab4d6003f225fc28f1b18 Mon Sep 17 00:00:00 2001
From: mintsuki <mintsuki@protonmail.com>
Date: Sat, 29 Jan 2022 11:54:38 +0100
Subject: [PATCH] chainload: Assume boot drive if DRIVE parameter is omitted

---
 CONFIG.md                  |  4 ++--
 stage23/protos/chainload.c | 17 +++++++++--------
 2 files changed, 11 insertions(+), 10 deletions(-)

diff --git a/CONFIG.md b/CONFIG.md
index ae2c5591..5259f53f 100644
--- a/CONFIG.md
+++ b/CONFIG.md
@@ -123,8 +123,8 @@ Some keys take *URIs* as values; these are described in the next section.
   * `KASLR` - For relocatable kernels, if set to `no`, disable kernel address space layout randomisation. KASLR is enabled by default.
   * `TEXTMODE` - If set to `yes`, prefer text mode if the kernel has no video mode requirements. (Only for stivale2)
 * Chainload protocol on BIOS:
-  * `DRIVE` - The 1-based BIOS drive to chainload.
-  * `PARTITION` - The 1-based BIOS partition to chainload, if omitted, chainload drive.
+  * `DRIVE` - The 1-based BIOS drive to chainload, if omitted, assume boot drive.
+  * `PARTITION` - The 1-based BIOS partition to chainload, if omitted, chainload drive (MBR).
 * Chainload protocol on UEFI:
   * `IMAGE_PATH` - URI of the EFI application to chainload.
   * `RESOLUTION` - The resolution to be used. This setting takes the form of `<width>x<height>x<bpp>`. If the resolution is not available, Limine will pick another one automatically. Omitting `<bpp>` will default to 32.
diff --git a/stage23/protos/chainload.c b/stage23/protos/chainload.c
index f76a1b21..15960789 100644
--- a/stage23/protos/chainload.c
+++ b/stage23/protos/chainload.c
@@ -87,20 +87,21 @@ void chainload(char *config) {
     int drive; {
         char *drive_config = config_get_value(config, 0, "DRIVE");
         if (drive_config == NULL) {
-            panic(true, "chainload: DRIVE not specified");
+            drive = boot_volume->index;
+        } else {
+            val = strtoui(drive_config, NULL, 10);
+            if (val < 1 || val > 256) {
+                panic(true, "chainload: BIOS drive number outside range 1-256");
+            }
+            drive = val;
         }
-        val = strtoui(drive_config, NULL, 10);
-        if (val < 1 || val > 256) {
-            panic(true, "chainload: BIOS drive number outside range 1-256");
-        }
-        drive = val;
     }
 
+    struct volume *p = volume_get_by_coord(false, drive, part);
+
     size_t rows, cols;
     init_vga_textmode(&rows, &cols, false);
 
-    struct volume *p = volume_get_by_coord(false, drive, part);
-
     volume_read(p, (void *)0x7c00, 0, 512);
 
     spinup(p->drive);